An electric motor (hereinafter, referred to as “motor”) includes a stator that generates a rotating magnetic field and a rotor that can rotate around its rotational shaft while interacting with the generated rotating magnetic field. The rotating magnetic field can be generated when electric power is supplied to a wiring wound around the stator. In a case where the electric power supplied to the wiring is three-phase AC power, the electric motor is referred to as a three-phase AC motor.
A three-phase AC motor is conventionally used to drive a main spindle or a feed shaft of a machine tool. In particular, a three-phase AC synchronous motor is commonly used as a driving source because it can accurately control the angular position or the rotational speed of a rotor. Hereinafter, the three-phase AC synchronous motor is described in detail below.
FIG. 1 is a cross-sectional view illustrating essential parts of a conventional electric motor 900. The electric motor 900 includes a stator 90 and a rotor 93. The stator 90 has a cylindrical body, which has a cylindrical inner surface on which a plurality of polar teeth T1 to T18 are arrayed in the circumferential direction. In the illustrated example, the number of the polar teeth T1 to T18 is eighteen (18). Each space between two neighboring polar teeth is referred to as a slot. As illustrated in the drawing, the stator 90 has eighteen slots S1 to S18. Windings 91, disposed along a winding path that goes into each slot, are wound around each polar tooth in such a way as to form a magnetic pole. The illustrated windings 91 of the electric motor 900 have a concentrated winding, which is characterized by winding the wire around one polar tooth in a concentrated fashion.
The winding configuration of the windings 91 is described in detail below with reference to FIG. 2 and FIG. 3. FIG. 2 schematically illustrates an electric motor. In the drawing, a portion indicated by M is the electric motor 900. Three lines extending outward from the portion M are three-phase leads of the electric motor 900. Each helical line in the portion M is a winding provided in the electric motor 900. A winding configuration of a U-phase winding (i.e., one of the three-phase windings), which is positioned between U and X windings, is described in detail below with reference to FIG. 3. The winding 91 is wound three turns around each of three consecutive polar teeth T2, T3, and T4 successively, to form three coils C1, C2, and C3. Further, the winding 91 is wound three turns around each of three consecutive polar teeth T11, T12, and T13, which are spaced from the polar teeth T2, T3, and T4, to form three coils C4, C5, and C6 at respective polar teeth T11, T12, and T13. The number of times the winding is wound around one polar tooth is generally referred to as a “number of turns.” The number of turns of each coil illustrated in FIG. 3 is three (3). Each of the V-phase and W-phase windings is similar to the U-phase winding in that the winding 91 is successively wound around each of three consecutive polar teeth and then successively wound around another three polar teeth, which are spaced from the preceding three polar teeth, although the polar teeth are not the same. As a result, U-phase, V-phase, and W-phase concentrated winding coils can be formed in increments of three polar teeth.
On the other hand, the rotor 93 includes a ring 95 coupled with a magnetic member 94 and a plurality of permanent magnets 96 that are fixed to the magnetic member 94 (see FIG. 1). The above-mentioned electric motor 900 is characteristic in that a plurality of coils of the same phase are continuously disposed in the circumferential direction. Therefore, the magnetic flux to be formed when electric power is supplied to the winding 91 has a trapezoidal distribution, according to which the magnitude of the magnetic flux is substantially constant in most of the entire range of consecutive coils and suddenly attenuates at the end of the consecutive coils. The trapezoidal distribution is not desirable in that a torque ripple tends to occur.
To reduce the torque ripple, it may be effective to optimize the shape of each polar tooth that constitutes the stator, as discussed in Japanese Utility Model Publication Laid-Open No. Hei 2-30270. More specifically, a front end surface of each polar tooth that faces the rotor is curved in such a way as to set the distance between the polar tooth and the rotor to be shorter at the center, and longer at each end of the front end surface in the circumferential direction. However, increasing the distance between the front end of a polar tooth and the rotor is not desired because the magnetic force acting between the polar tooth and the rotor is weakened undesirably. Further, as discussed in Japanese Patent Publication Laid-Open No. Hei 11-308795, a skew structure may be employable. More specifically, the skew structure includes a groove formed on an outer cylindrical surface of a cylindrical rotor core in such a way as to incline relative to a rotational shaft. However, the skew structure is not desired in that the torque constant tends to decrease. Further, a distributed winding (i.e., a winding different from the concentrated winding) may be employable.
As mentioned above, the number of turns of the coil illustrated in FIG. 3 is three. The number of turns of an electric motor is determined taking demanded performance specifics into consideration. In general, it is known that the voltage to be generated across a winding of the electric motor is proportional to the number of turns of the winding and a temporal change amount of the magnetic flux that intersects the winding. Therefore, in a case where the current supply to an electric motor is constant, the torque to be generated by the motor increases in accordance with the increase in the number of turns. On the other hand, the voltage to be generated across the winding increases in accordance with the increase in the number of turns. The voltage to be generated across the winding becomes larger in proportion to the rotational speed of the electric motor. Therefore, when the number of turns increases, the voltage to be generated across the winding increases when the rotational speed is high. If the voltage to be generated across the winding reaches a power source voltage of an amplifier that supplies electric power to the electric motor, the electric motor cannot be driven because it is unfeasible to supply current from the amplifier to the electric motor. Accordingly, the number of turns of a winding is determined so that a desired power (i.e., a product of the rotational speed and the torque) can be obtained when the current is supplied in a range where the voltage to be generated across the winding does not exceed a permissible value having been determined beforehand.
In general the number of turns of a coil is an integer and takes a discrete value. Therefore, it may be difficult to optimize a relationship between the output and the voltage to be generated across the wiring. The relationship between the power and the voltage to be generated across the winding is described in detail below with reference to FIG. 4 and FIG. 5. FIG. 4 illustrates a relationship between a base rotational speed nb, a maximum rotational speed nt, and an output p0 required to attain the demanded performance. In a case where the number of turns of a coil is two (2), it is feasible to set the voltage to be generated across the winding to be equal to or less than the permissible value in the entire rotational range (from 0 to nt). However, the torque to be generated is smaller and the required output p0 cannot be obtained at the base rotational speed nb. In FIG. 5, a dotted line indicates output characteristics obtainable in this case. If the number of turns is increased to three (3) to attain the required output p0, the voltage to be generated across the winding may exceed the permissible value in a higher speed region. Therefore, it is necessary to reduce the current to be supplied to the winding in such a way as to reduce the voltage to be generated across the winding in a speed region exceeding the permissible value (i.e., rotational speed nc or more). As a result, the required output cannot be obtained in a higher speed region. In FIG. 5, a solid line indicates output characteristics obtainable in this case. The required performance illustrated in FIG. 4 may be attained if it is feasible to set the number of turns to be a value between two (2) and three (3). As discussed in Japanese Patent Publication Laid-Open No. 2012-135133, it is conventionally known to set the number of turns to be a non-integer value in a distributed winding type winding.
With the concentrated winding type winding, it is easy to perform coil forming operations, compared to the distributed winding type winding. However, the concentrated winding type winding tends to cause a torque ripple. Further, if the number of turns of the winding is an integer, the demanded performance specifics cannot be sufficiently attained, as will be understood from the relationship between the output at a lower rotational speed and the voltage to be generated in the winding at a higher rotational speed.